Optical head device, optical information device, computer, disc player, car navigation system, optical disc recorder, and vehicle

ABSTRACT

An optical head device compatible to different types of optical discs and capable of guaranteeing a sufficiently wide dynamic range for a low density optical disc, and the like are provided. 
     The optical head device includes a plurality of light sources switchably usable; an objective lens for converging light emitted from one of the plurality of light sources to an information recording layer of an optical disc; and a light detector for receiving the light reflected by the information recording layer and outputting an electric signal based on the amount of the received light. The plurality of light sources include a first light source for emitting light having a first wavelength and a second light source for emitting light having a second wavelength shorter than the first wavelength. In the optical head device, a defocus detection range of a focusing error signal obtained based on the light having the first wavelength is set to be wider than a defocus detection range of a focusing error signal obtained based on the light having the second wavelength.

TECHNICAL FIELD

The present invention relates to an optical information apparatus forreproducing information from, or recording information to, aninformation recording medium exemplified by an optical disc, an opticalhead device for reproducing or recording information in such an opticalinformation apparatus, and an information apparatus and system using thesame.

BACKGROUND ART

A digital versatile disc (DVD) allows digital information to be recordedthereon at a recording density about six times higher than a compactdisc (CD), and is known as an optical disc capable of storing largecapacity data. Along with the recent increase of the informationquantity to be recorded on optical discs, an optical disc having alarger recording capacity is desired.

In order to increase the recording capacity of an optical disc, therecording density of information needs to be increased. Specifically,the size of an optical spot formed by light which irradiates an opticaldisc for recording information to the optical disc or reproducinginformation recorded on the optical disc needs to be decreased. Forrealizing this, it is necessary to decrease the wavelength of laserlight from a light source and to increase the numerical aperture (NA) ofan objective lens.

For DVDs, a light source for emitting light having a wavelength of 660nm and an objective lens having a numerical aperture (NA) of 0.6 areused. For, for example, Blu-ray discs (BD) having a larger recordingcapacity, blue laser light having a wavelength of 405 nm and anobjective lens having an NA of 0.85 are used. Thus, a recording densityfive times higher than that of an existing DVD is achieved.

For an optical information apparatus for performing high densityrecording and reproduction using laser light having a short wavelength,such as blue laser light or the like, compatibility with existingoptical discs can improve usefulness thereof as an apparatus andincrease cost performance. Conventionally, an optical head device asdescribed below is available for realizing an optical informationapparatus capable of compatibly performing recording to, or reproductionfrom, a plurality of optical discs of different recording densities.

FIG. 17 shows an example of a structure of a conventional optical headdevice.

A light beam 210 emitted from a light source 201 is transmitted througha polarization anisotropic hologram 202. The light beam 201 is convertedinto parallel light by a collimator lens 203 and is changed into lightof circular polarization by a ¼ wavelength plate 204. A first objectivelens 205 converges the light beam 210 on an information recording layerof an optical disc 327.

For an optical disc 328 of a lower recording density than that of theoptical disc 327, only an inner part of the light beam close to anoptical axis thereof is converged by a second objective lens 250 havinga smaller numerical aperture (as represented with the dotted line). Thefirst and second objective lenses are mechanically exchanged inaccordance with the type of the optical disc. Where a single objectivelens is used, a variable aperture is used to change the numericalaperture for the optical disc.

The light of circular polarization which is reflected by the opticaldisc follows the same optical path in the opposite direction. Duringthis process, the light is converted by the ¼ wavelength plate 204 intolight of linear polarization of a direction perpendicular to the lightemitted from the light source 201. Therefore, the light is diffracted bythe polarization anisotropic hologram 202 to be incident on lightdetectors 263 and 266. Based on signals obtained from the lightdetectors, the objective lens is moved along the optical axis to performfocusing control.

When the effective diameter of the light beam is reduced in order toapply a smaller numerical aperture for a disc of a lower recordingdensity, the diffracted light is made smaller as diffracted light 213and 214 (dotted lines) on the light detectors shown in a bottom part ofFIG. 17.

FIG. 18 shows a waveform of a focusing error signal obtained from aconventional optical head device. A focusing error signal 3281 isreduced in the inclination angle in the vicinity of the center of thegraph; namely, the sensitivity thereof is decreased. In order to avoidsuch a decrease of the sensitivity in the vicinity of the center, asshown the bottom part of FIG. 17, the width of divided areas of thelight detectors is partially changed and the signal to be used is alsochanged in accordance with the change of the numerical aperture. Bychanging the numerical aperture in this manner, a focusing error signal3271 with a constant sensitivity can be obtained.

Patent Document 1: Japanese Laid-Open Patent Publication No. 2000-207769

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Among optical discs practically used so far, newer optical discs are ofa higher recording density. Historically, the recording density has beenincreased from low to high in the order of compact disc (CD), DVD,HD-DVD, Blu-ray disc (BD).

Earlier optical discs were developed with a lower production technology.Therefore, CDs are distorted in shape more often and at a higher extentthan DVDs or BDs. In other words, an optical disc of a higher recordingdensity is produced at a higher flatness in order to realize higherprecision.

A disc of a lower recording density is quite distorted, and so when sucha disc is mounted on an optical information apparatus and rotated by aspindle motor, the level of the information recording layer moves up anddown. This is generally called “face movement”.

In order to start a focus servo loop with certainty for a disc which maycause a large face movement, a focusing error signal having a widedynamic range needs to be used. Specifically, referring to FIG. 18, itis desirable that defocus point A, at which the focusing error signalhas the maximum strength, and defocus point B, at which the focusingerror signal has the minimum strength, have a large separationtherebetween.

Meanwhile, information recording on BDs needs to be done at a highdensity almost 40 times higher than CDs, using blue laser light and ahigh numerical aperture (NA: 0.85). Therefore, the optical spot formedon the information recording layer of such an optical disc is smallerthan that for a CD, and the focal depth is shallower than that for a CD.This requires focus servo (focus control) to be done more precisely. Forrealizing this, referring to FIG. 18, it is necessary that the defocuspoints A and B, at which the focusing error signal has the maximumstrength and the minimum strength respectively, have a small separationtherebetween so that the sensitivity between the defocus points A and Bis high.

The above-described example of the conventional art is developed for thepurpose of keeping focusing error signals linear and providing focusingerror signals with the same sensitivity and the same dynamic range fordifferent types of optical discs. Accordingly, a method for merelyrealizing focusing error signals having the same characteristics isprovided. In the above example, there is no recognition found that alarger dynamic range is needed for CDs whereas a higher sensitivity isneeded for BDs.

The above example of the conventional art has another problem asfollows. This example only uses a single light source, and does notprovide any appropriate structure for realizing, in an optical headdevice having a plurality of light sources, especially light sources forthree different wavelengths, a required focusing error signal regardlessof which wavelength is to be used, at the lowest possible cost.

An object of the present invention is to provide an optical head devicewhich is compatible to different types of optical discs, guarantees asufficiently wide dynamic range for lower density optical discs, and isproduced at low cost.

Means for Solving the Problems

An optical head device according to the present invention includes aplurality of light sources switchably usable; an objective lens forconverging light emitted from one of the plurality of light sources toan information recording layer of an optical disc; and a light detectorfor receiving the light reflected by the information recording layer andoutputting an electric signal based on the amount of the received light.The plurality of light sources include a first light source for emittinglight having a first wavelength and a second light source for emittinglight having a second wavelength shorter than the first wavelength; anda defocus detection range of a focusing error signal obtained from anelectric signal based on the amount of received light having the firstwavelength is wider than a defocus detection range of a focusing errorsignal obtained from an electric signal based on the amount of receivedlight having the second wavelength.

The light having the first wavelength reflected by the informationrecording layer and the light having the second wavelength reflected bythe information recording layer may be both incident on the lightdetector; and the light detector may perform photoelectric conversion onthe incident light to generate an electric signal for obtaining afocusing error signal.

The optical head device may further include an optical element locatedon an optical path on which the light reflected by the informationrecording layer proceeds until being incident on the light detector, theoptical element giving astigmatism to light transmitting therethrough;and the light detector may receive the light reflected by theinformation recording layer and given the astigmatism to generate thefocusing error signal by an astigmatism method.

The optical head device may further include a rising mirror for turningthe light emitted from one of the plurality of light sources in adirection vertical to the optical disc; and an optical axis of the lightincident on the rising mirror may have an angle of about 45 degrees withrespect to a track groove of the optical disc.

The light having the first wavelength emitted from the first lightsource and the light having the second wavelength emitted from thesecond light source may be both incident on the objective lens.

The objective lens may include at least an inner zone including anoptical axis and an outer zone surrounding the inner zone, and the innerzone may include a central area including the optical axis and an outerperipheral area outer with respect to the central area; and the lighthaving the first wavelength passing through the central area may beconverged to a first position on the information recording layer of theoptical disc, and the light having the first wavelength passing throughthe outer peripheral area may be given a spherical aberration to beconverged to at least one second position, which is different from thefirst position in a direction vertical to the optical disc.

The light having the first wavelength passing through the outerperipheral area may be converged to a position farther from theobjective lens than the first position by the spherical aberration.

The plurality of light sources may further include a third light sourcefor emitting light having a third wavelength shorter than the secondwavelength; the objective lens may converge the light, having the firstwavelength passing through the central area of the inner zone, through atransparent substrate of the first optical disc; may converge the light,having the third wavelength passing through an area through the centralarea and the outer zone surrounding the inner zone, through atransparent substrate of the third optical disc; and may converge thelight, having the second wavelength passing through an effectivediameter zone of the objective lens, through a transparent substrate ofthe second optical disc.

The inner zone of the objective lens may be designed such that anaverage value of sums of squares of the aberrations of the light havingthe first wavelength passing through the inner zone is equal to or lessthan 20 mλ.

A focal distance f1 of the objective lens for converging the lighthaving the first wavelength may be longer than a focal distance f2 ofthe objective lens for converging the light having the secondwavelength.

The light having the first wavelength reflected by the informationrecording layer and the light having the second wavelength reflected bythe information recording layer may be both incident on the lightdetector; and the light detector may perform photoelectric conversion onthe incident light to generate an electric signal for obtaining afocusing error signal.

The optical head device may further include a parallel plate forreflecting the light having the second wavelength emitted from thesecond light source; and an optical element for reflecting the lighthaving the second wavelength reflected by the parallel plate andtransmitting the light having the first wavelength emitted from thefirst light source.

An objective lens according to the present invention is used, in anoptical head device including a plurality of light sources switchablyusable, for converging light emitted from one of the plurality of lightsources to an information recording layer of an optical disc. Theplurality of light sources of the optical head device further include afirst light source for emitting light having a first wavelength and asecond light source for emitting light having a second wavelengthshorter than the first wavelength; the objective lens includes at leastan inner zone including an optical axis and an outer zone surroundingthe inner zone; and the light having the first wavelength passingthrough an central area is converged to a first position on theinformation recording layer of the optical disc, and the light havingthe first wavelength passing through an outer peripheral area is given aspherical aberration to be converged to at least one second position,which is different from the first position in a direction vertical tothe optical disc.

The light having the first wavelength passing through the outerperipheral area may be converged to a position farther from theobjective lens than the first position by the spherical aberration.

In the case where the plurality of light sources of the optical discdevice further include a third light source for emitting light having athird wavelength shorter than the second wavelength, the objective lensmay converge the light, having the first wavelength passing through thecentral area of the inner zone, through a transparent substrate of thefirst optical disc; may converge the light, having the third wavelengthpassing through the central area and the outer zone surrounding theinner zone, through a transparent substrate of the third optical disc;and may converge the light, having the second wavelength passing throughan effective diameter zone of the objective lens, through a transparentsubstrate of the second optical disc.

The inner zone may be designed such that an average value of sums ofsquares of the aberrations of the light having the first wavelengthpassing through the inner zone is equal to or less than 20 mλ.

A focal distance f1 for converging the light having the first wavelengthmay be longer than a focal distance f2 for converging the light havingthe second wavelength.

An optical information apparatus according to the present inventionincludes the above-described optical head device; a motor for rotatingthe optical disc; and a circuit for controlling and driving the motor,an optical lens and the light source based on a signal obtained from theoptical head device.

A computer according to the present invention includes theabove-described optical information apparatus; an input device or aninput terminal for inputting information; a calculation device forperforming a calculation based on at least one of information input fromthe input device and information reproduced by the optical informationapparatus; and an output device or an output terminal for displaying oroutputting at least one of information input from the input device,information reproduced by the optical information apparatus, and acalculation result obtained by the calculation device.

An optical disc player according to the present invention includes theabove-described optical information apparatus; and aninformation-to-image decoder for converting an information signalobtained by the optical information apparatus into an image.

A car navigation system according to the present invention includes theabove-described optical information apparatus; an information-to-imagedecoder for converting an information signal obtained by the opticalinformation apparatus into an image; and a positional sensor.

An optical disc recorder according to the present invention includes theabove-described optical information apparatus; and an encoder forconverting image information into information of a format recordable bythe optical information apparatus. The post-conversion image informationis recorded on an optical disc.

A vehicle according to the present invention includes a vehicle bodyhaving the above-described optical information apparatus mountedthereon; and a power generation section for generating power for movingthe vehicle body.

EFFECTS OF THE INVENTION

According to an optical head device of the present invention, wheninformation is reproduced from, or recorded to, a high density opticaldisc using an objective lens having a large numerical aperture (NA), ahigh sensitivity focusing error signal can be obtained. Therefore,recording or reproduction can be stably performed with highly precisefocus servo. With the same optical head device, when information isreproduced from, or recorded to, a low density optical disc, a focusingerror signal having a wide dynamic range can be obtained. Therefore,even where there is a shape error, focus servo can be performed withcertainty.

According to an optical head device of the present invention, an opticalsystem including a plurality of light sources to be compatible to aplurality of types of optical discs can be provided with a small numberof components, in a simple manner and at low cost.

An optical information apparatus using an optical head device accordingto the present invention enables information to be stably reproducedfrom a larger capacity optical disc or a plurality of optical discs ofdifferent recording densities with a single optical head device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a structure of an optical head device 100 according toEmbodiment 1.

FIG. 2 shows an example of an objective lens 9.

FIG. 3 shows an example of dividing a light detection area of a lightdetector 10, which is preferable to detect a focusing error signal by anastigmatism method.

FIG. 4( a) shows a light beam of infrared light transmitting through theobjective lens 9, and FIG. 4( b) shows an effective diameter of theobjective lens 9 for blue light as seen from a direction of an opticalaxis.

FIG. 5 shows a waveform of a focusing error signal obtained when a BD isirradiated with blue light from alight source 1.

FIG. 6 shows a waveform of a focusing error signal obtained when a DVDis irradiated with red light from alight source 12.

FIG. 7 shows a waveform of a focusing error signal obtained when a CD isirradiated with infrared light from the light source 12.

FIG. 8 shows a structure of an optical head device 200 according toEmbodiment 2.

FIG. 9 shows a focusing error signal for a CD, in the optical headdevice 200 using the objective lens 25, obtained by calculation.

FIG. 10 shows a reference example of an optical head using a paraxiallens 511.

FIG. 11 shows a focusing error signal for a CD, in an optical headdevice shown in FIG. 10, obtained by calculation.

FIG. 12 shows a structure of an optical information apparatus 167according to Embodiment 3 including the optical head device according toEmbodiment 1 or 2.

FIG. 13 shows a structure of a computer (PC) 300 according to Embodiment4 including the optical information apparatus 167 according toEmbodiment 3.

FIG. 14 shows a structure of an optical disc player 321 according toEmbodiment 5 including the optical information apparatus 167 accordingto Embodiment 3.

FIG. 15 shows a structure of an optical disc recorder 110 according toEmbodiment 6 including the optical information apparatus 167 accordingto Embodiment 3.

FIG. 16 shows a structure of a vehicle 300 according to Embodiment 7including the optical information apparatus 167 according to Embodiment3.

FIG. 17 shows an example of a structure of a conventional optical headdevice.

FIG. 18 shows a waveform of a focusing error signal obtained in aconventional optical head device.

DESCRIPTION OF THE REFERENCE NUMERALS

-   -   1, 12 Light source module    -   2, 14, 15 Light beam    -   3 Parallel plate    -   4 Hologram element    -   5 Relay lens    -   6 Wedge    -   7 Collimator lens    -   8 Rising mirror    -   9, 25 Objective lens    -   10 Light detector    -   13 Diffraction element    -   18 ¼ wavelength plate    -   40 Inner zone of the objective lens 9    -   41 Middle zone of the objective lens 9    -   42 Outer zone of the objective lens 9    -   43 Infrared light passing through an central area of the inner        zone area 40    -   44 Infrared light passing through an outer peripheral area of        the inner zone area 40    -   100, 200 Optical head device

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to the attached drawings, embodiments of the presentinvention will be described.

Embodiment 1

FIG. 1 shows a structure of an optical head device 100.

The optical head device 100 includes semiconductor laser elements(hereinafter, referred to as “light sources”) for emitting light beamsof at least two different wavelengths, and is capable of recordinginformation to, and/or reproducing information from, different types ofoptical discs. In the following description, there are three lightsources for recording information to, and/or reproducing informationfrom, a BD, a DVD and a CD.

The optical head device 100 includes light source modules 1 and 12, aparallel plate 3, a hologram element 4, a relay lens 5, a wedge 6, acollimator lens 7, a rising mirror 8, an objective lens 9, a lightdetector 10, a diffraction element 13, and a ¼ wavelength plate 18.

Hereinafter, functions of these elements will be described through adescription of an operation of the optical head device 100 for providinglight. In FIG. 1, Y axis is vertical to an optical axis of the objectivelens 9 and is generally vertical to a direction of track grooves of anoptical disc (not shown), and Z axis is in a direction of the opticalaxis of the objective lens 9, namely, the focusing direction (verticalto the sheet of FIG. 1). Y axis is in a direction in which the opticalhead device is to be moved for recording information to, or reproducinginformation from, a position in an inner zone or a position in an outerzone of the optical disc. X axis is vertical to Z axis and Y axis, andis generally parallel to the direction of the track grooves of anoptical disc as seen from the position of the objective lens 9. Theoptical head device 100 may have a mirror reverse structure in which Xaxis and Y axis are exchanged, or a structure which is rotated from thestate in FIG. 1 by 90 degree, 180 degrees or 270 degrees.

The light source module 1 includes a semiconductor laser element whichis a short wavelength light source (for example, a blue light source).For BDs, a light beam 2 is emitted from the blue light source of thelight source module 1.

The light beam 2 of linear polarization emitted from the shortwavelength light source is reflected by a polarization separation filmat a surface of the parallel plate 3 and is transmitted through thehologram element 4. The light beam 2 transmitted through the hologramelement 4 is converted by the relay lens 5 into a luminous fluxdiverging more widely. The relay lens 5 has a concave lens function. Therelay lens 5 converts a small point ahead angle from the aperture of theobjective lens 9 toward the light source module 1, i.e., a smallnumerical aperture (NA) on the side of, and in the vicinity of, thelight source into a large NA on the side of the collimator lens 7.

Next, the collimator lens 7 changes the parallel degree of the lightbeam 2. For example, the collimator lens 7 converts a light beam 14 togenerally parallel light. The collimator lens 7, which is for reducingthe parallel degree, i.e., reducing the diverging degree, may be formedof a combination of two lenses. In the case where the collimator lens 7is formed of two lenses, when the collimator lens 7 is moved along anoptical axis thereof in order to correct the spherical aberration, onlyone of the two lenses may be moved. The ¼ wavelength plate 18 changesthe linear polarization into circular polarization. The rising mirror 8turns the optical axis of the light from the collimator lens 7 in adirection of Z axis, which is perpendicular to the optical disc. In thisembodiment, the optical axis of the light incident on the rising mirror8 has an angle of about 45 degrees, or more roughly, an angle in a rangeof 35 degrees to 55 degrees, with respect to the track grooves of theoptical disc. This provides an advantage as described later. Forexample, the tangential line of the track grooves of the optical disc isparallel to Y axis.

As described later, when light is branched by the parallel plate 3 to bedirected toward the light source module 1 and the light detector 10, thelight transmitted through the parallel plate 3 is provided withastigmatism. When the angle between the optical axis of the lightincident on the rising mirror 8 and the track grooves is changed from 35degrees to 55 degrees, the angle between the focal line of theastigmatism and the track grooves is changed from 35 degrees to 55degrees. Thus, by providing four divided areas of the light detector 10to allow light having astigmatism to be incident thereon and usingsignals obtained from the four divided areas, a focusing error signaland also a tracking error signal by a known push-pull method ordifferential phase detection method can be obtained.

The objective lens 9 converges the light beam 2 on an informationrecording layer of a high density optical disc such as a BD or the likethrough a transparent substrate having a thickness smaller than 0.6 mm,for example, about 0.1 mm. The objective lens 9 will be described laterin detail.

The light beam reflected by the information recording layer of theoptical disc follows the same optical path in the opposite direction.The ¼ wavelength plate 18 changes the reflected light beam into light oflinear polarization which is vertical to the polarization direction ofthe light beam advancing toward the optical disc. The light of thelinear polarization is reflected by the wedge 6 and is transmittedthrough the relay lens 5 and the hologram element 4. The hologramelement 4 diffracts a part of the beam. The light beam transmittedthrough the hologram element 4 and the partial light diffracted by thehologram element 4 are transmitted through the parallel plate 3 havingthe polarization separation film on a surface thereof, and are branchedin a different direction from the direction toward the light sourcemodule 1, to be incident on the light detector 10. The light detector 10performs photoelectric conversion on the incident light to generate anelectric signal for obtaining an information signal and a servo signal.A “servo signal” is a general term referring to a focusing error signalfor focus control, i.e., focus servo or a tracking signal for trackingcontrol.

A reflective hologram (not shown) may be provided at a position which isoff the optical axis of the hologram element 4 and does not shield thelight incident on the objective lens 9. By receiving diffracted lightreflected by such a reflective hologram by the light detector 10 andmonitoring the light intensity of the light beam 2, a monitoring signalfor stabilizing the light intensity can be obtained without increasingthe number of components.

Now, a light beam output from the light source module 12 will bedescribed.

The light source module 12 includes a semiconductor laser element whichis, for example, an infrared light source. For an optical disc of alower recording density than that of a BD (for example, a compact disc(CD)), a light beam 14 is emitted by the infrared light source of thelight source module 12. The light beam 14 emitted by the infrared lightsource is transmitted through a diffraction element 13 which diffracts apart of the light in order to form a sub spot on the optical disc(partial diffraction), and is transmitted through the wedge 6 having awedge-shaped cross-section. Then, the light is incident on thecollimator lens 7.

Next, the light beam 14 incident on the collimator lens 7 is changed inthe parallel degree. For example, the light beam 14 is converted intogenerally parallel light by the collimator lens 7.

Then, the polarization direction of the light beam 14 is converted bythe ¼ wavelength plate 18. The optical axis of the resultant light isbent by the rising mirror 9 to a direction perpendicular to an opticaldisc of a lower recording density than that of the BD (for example, acompact disc (CD)). The objective lens 9 converges the light beam 14 onan information recording layer of the optical disc through a transparentsubstrate having a thickness of about 1.2 mm.

The light beam reflected by the information recording layer of theoptical disc follows the same optical path in the opposite direction andis branched by a polarization selective film provided on a surface ofthe collimator lens 7 in a different direction from the direction towardthe light source module 12, to be incident on the light detector 10 asin the case of the light beam 2. In an optical head device such as areproduction-only optical head device or the like, in which the lightutilization factor may be low, a non-polarization branching film may beused instead of the polarization selective film.

The light detector 10 performs photoelectric conversion on the incidentlight to generate an electric signal for obtaining an information signaland a servo signal. In the case where an amplifier circuit is built inthe light detector 10, a good information signal having a highsignal-to-noise ratio (S/N ratio) can be obtained while a thin, compactand stable optical head device can be realized.

The light source module 12 also includes a semiconductor laser elementwhich is, for example, a red light source. The red light source is usedfor reproducing information from, or recording information to, anoptical disc of a middle recording density between the above-describedtwo types of optical discs (for example, a DVD).

The light source 12 includes a red light source and an infrared lightsource in this embodiment, but these light sources may be providedseparately. In such a case, the red light source may be located in thevicinity of the infrared light source and a beam splitter may beprovided for aligning an optical path for the red light source and anoptical path for the infrared light source. However, provision of a beamsplitter increases the number of component. Therefore, it is preferableto provide the red light source and the infrared light source in thelight source module 12 in order to suppress the increase of the numberof components, which would be caused by providing the beam splitter. Inthis embodiment, the red light source or the infrared light source isadditional, and the effects of the present invention described later canbe obtained as long as at least either one of the red light source andthe infrared light source is provided in addition to the blue lightsource.

A light beam 15 of red light emitted from the light source module 12follows substantially the same optical path as that of the infraredlight to reach the objective lens 9 and is converged by the objectivelens 9 on an information recording layer of the optical disc such as aDVD or the like through a transparent substrate having a thickness ofabout 0.6 mm. Then, the light beam reflected by the informationrecording layer of the optical disc follows the same optical path in theopposite direction to be incident on the light detector 10. The lightdetector 10 performs photoelectric conversion on the incident light togenerate an electric signal for obtaining an information signal and aservo signal.

Generally in order to branch an optical path, a cubic beam splitterobtained by attaching two triangular transparent elements together maybe used. However, using a parallel plate and a wedge as in the presentinvention reduces the number of components and so their costs. It shouldbe noted that in the case where a single beam splitter is provided on anon-parallel optical path from the light source to the objective lensfor light transmission, it is desirable to use a wedge like the wedge 6in this embodiment and set the incident angle of the optical axis tosmaller than 45 degrees, in order to prevent occurrence of astigmatism.

Even with such considerations, astigmatism may occur due to productionerrors or the like. Accordingly, in the example of the present inventionshown in FIG. 1, the light beam 2 to be converged on the optical disc ofthe highest recording density is reflected by, instead of beingtransmitted through, the two beam splitters on the non-parallel opticalpath from the light source 1 to the collimator lens 7. This provides aneffect of realizing good signal reproduction or signal recording evenfor an optical disc of the highest recording density such as a BD or thelike.

The objective lens 9 is fixed at a prescribed position in an actuator(not shown) for fine-moving the objective lens 9. The actuator iscapable of fine-moving the objective lens 9 both in a focusing direction(Z axis direction) perpendicular to the information recording layer ofthe optical disc and a tracking direction (Y axis direction) of theoptical disc.

Where an objective lens having an NA of 0.85 or larger is used as theobjective lens 9 for reproducing information from, or recordinginformation to, a BD or the like, a conspicuous spherical aberrationoccurs due to the thickness of the transparent substrate existentbetween the light incidence face and the information recording layer ofthe optical disc because the numerical aperture is large.

In this embodiment, a driving device 11 and a driving mechanism are usedto move the collimator lens 7 in the optical axis direction thereof, sothat the diverging or converging degree of the light proceeding from thecollimator lens 7 toward the objective lens 9 is changed. When thediverging or converging degree of the light incident on the objectivelens changes, the spherical aberration changes. Utilizing this, thespherical aberration caused by the difference in the substrate thicknessis corrected. As the driving device 11, a stepping motor or a brushlessmotor is usable, for example. The driving mechanism includes a holder 17for holding the collimator lens 7, guide shafts 16 for guiding themovement of the holder 17, and a gear (not shown) for conveying thedriving force of the driving motor 11 to the holder 17. The holder 17for holding the collimator lens 7 may be integrated with the collimatorlens 7, in which case the number of components can be reduced.

In the present invention, the optical axis of the collimator lens 7 isnot parallel to Y axis. This prevents the collimator lens 7 from makingan unintentional movement against the inertial force generated byacceleration or deceleration when the entire optical head device ismoved inward along the optical disc.

An optical recording system using a blue light source or a red lightsource is provided for the purpose of performing large capacityrecording or reproduction by increasing the recording density though useof a short wavelength. In order to handle large capacity information,the recording or reproduction speed of information needs to beincreased. Especially for recording, the light emission intensity needsto be changed at a high speed for high speed recording. Namely, in orderto modulate the light intensity at a high speed, the electric currentflown to emit light from a blue LD light source needs to be modulated ata high speed. For this purpose, a current control circuit or a largescale integration (LSI) needs to be located in the vicinity of the lightsource. Circuits for controlling a light emitting current are largelycommon for blue light and red light, and so need to be provided as asingle LSI in order to reduce the size of the circuit.

In summary, it is desirable to provide a single LSI for controlling alight emitting current and locate a blue red source and a red lightsource in the vicinity thereof. It is also desirable to locate theseelements in correspondence with the outer zone of the optical disc inorder to enable recording or reproduction to be done in the inner zoneof the optical disc, up to a closest possible position to the center ofthe disc.

It is desirable that the objective lens 9 is located on a line whichgenerally passes the center of the optical disc, on which the opticalhead device is to be moved inward along the optical disc by a seekoperation. Thus, it is made possible to form a sub beam by a diffractiongrating 12 and detect a tracking signal by a three-beam method using thesub beam. As a result, stable signal detection can be performed.

FIG. 2 shows an example of the objective lens 9. The objective lens 9converges the infrared light 15 in an innermost zone thereof in thevicinity of the optical axis through a transparent substrate having athickness of about 1.2 mm of a low density optical disc 28 such as a CDor the like. The objective lens 9 converges the red light 14 expandingup to a middle zone thereof, which is larger than the inner zone to acertain extent, through a transparent substrate having a thickness ofabout 0.6 mm of an optical disc 27 such as a DVD or the like. Theobjective lens 9 converges the blue light 2 in an effective diameterzone thereof through a transparent substrate having a thickness of about0.1 mm or less of a high density optical disc 26 such as a BD or thelike.

In order to converge the light through transparent substrates havingdifferent thicknesses in this manner, it is effective to use adiffraction element as shown in FIG. 2. The diffraction element isprovided on the opposite side of the objective lens 9 to the side facingthe optical disc 26, 27 or 28.

The diffraction element is designed to be discontinuous among theinnermost zone, the middle zone and the zone outer thereof, so that theinnermost zone allows the light to be converged through any thickness ofthe substrate, whereas the outermost zone allows the light to beconverged through only a substrate having a thickness of 0.1 mm or less.Such designing is facilitated by using, for example, light sources fordifferent wavelengths as described above. For example, utilizing thatthe diffraction angle of the light diffracted by the diffraction gratingvaries among different wavelengths of light, i.e., infrared light for aCD, red light for a DVD and blue light for a BD, the sphericalaberration caused by the thickness difference of the substrate can becorrected or the aperture limitation can be switched as described abovein accordance with the type of disc.

Hereinafter, the term “design” used regarding an objective lensencompasses settings of optical performance of the diffraction element.

When the effective diameter of the objective lens 9 is changed asdescribed above, there is an undesirable possibility that the linearityof a focusing error signal is not guaranteed where a spot size methodusing a size change of the optical spot is used for detecting a focusingerror signal. One effective solution to this problem is using anastigmatism method. The reason is that the astigmatism method is fordetecting a shift of the focal point using a shape change of the opticalspot, instead of a size change of the optical spot.

FIG. 3 shows an example of dividing a light detection area of the lightdetector 10, which is preferable to detect a focusing error signal bythe astigmatism method. The light detector 10 includes light receivingareas 20, 21-1, 21-2, 23, 24-1 and 24-2. FIG. 3 shows a pattern of thelight receiving areas of the light detector 10 as seen from the sideopposite to the light incidence side. In FIG. 3, X axis and Z axis arethe same as those of FIG. 1.

The light receiving area 20 is provided for receiving blue light and redlight. The light receiving area 20 is divided into four, and a focusingerror signal is detected based on incident light using astigmatism givenby the parallel plate 3. A tracking signal is also obtained by aso-called differential phase detection method or push-pull method.

The light receiving areas 21-1 and 21-2 each receive a sub beamdiffracted by the diffraction grating 13 performed on the red lighttransmitting through the diffraction grating 13, and performsphotoelectric conversion on the incident light. Signals output from thelight receiving areas 21-1 and 21-2 are calculated together with apush-pull signal from the light receiving area 20 to be used for thedetection by a differential push-pull method. A focusing error signal isalso detected by the astigmatism method from the signals output from thelight receiving areas 21-1 and 21-2. By calculating such signalstogether with the focusing error signal obtained from the lightreceiving area 20, crosstalk from the tracking error signal can beremoved. A diffraction grating may be provided between the light source1 and the parallel plate 3, in which case a sub beam signal of the bluelight can be detected from the light receiving areas 21 and 22 as in thecase of the red light.

The light receiving areas 23, 24-1 and 24-2 are provided for receivinginfrared light. The distance between the central point of the lightreceiving area 20 and the central point of the light receiving area 23is set to be a value obtained by multiplying the distance between thelight emission point of red light and the light emission point ofinfrared light in the light source 12 by a magnification realized by therelay lens 5.

Where the distance between the central point of the light receiving area20 and the central point of the light receiving area 21-1 is L1, and thedistance between the central point of the light receiving area 23 andthe central point of the light receiving area 24-1 is L2, the ratiobetween L1 and L2 is set to be equal to the ratio between the wavelengthof the red light and the wavelength of the infrared light.

The light receiving area 23 is divided into four, and a focusing errorsignal is detected using astigmatism given by the parallel plate 3. Atracking signal is also obtained by a so-called differential phasedetection method or push-pull method. The light receiving areas 24-1 and24-2 each receive a sub beam diffracted by the diffraction grating 13performed on the infrared light transmitting through the diffractiongrating 13. By calculating signals output from the light receiving areas24-1 and 24-2 together with a push-pull signal from the light receivingarea 23, a tracking signal can be detected by the differential push-pullmethod.

Owing to a structure in which a plurality of light receiving areas areprovided on a single light detector or a single semiconductor chip toperform photoelectric conversion on light of different wavelengths, thenumber of semiconductor components can be reduced.

Now, a method for obtaining a focusing error signal having a widedetection area in a defocus direction for an optical disc which is of arelatively low recording density and is distributed on the market asindividual products causing a large face movement (for example, CD) willbe described.

FIG. 4( a) shows light beams 43 and 44 of infrared light transmittingthrough the objective lens 9. FIG. 4( b) shows an effective diameterzone of the objective lens 9 for blue light as seen from the opticalaxis direction.

As seen from the optical axis direction, the objective lens 9 includesan inner zone 40, a middle zone 41 and an outer zone 42. Each area isdesigned with a different design value (parameter).

The objective lens 9 converges the infrared light 15 transmittingthrough the inner zone 40 including the optical axis through atransparent substrate having a thickness of about 1.2 mm of the lowdensity optical disc 28 such as a CD or the like. The objective lens 9is designed to give infrared light transmitting through the middle zone41 and the outer zone 42 an aberration exceeding 70 mλms, which is theMarechal Criterion, or not to transmit infrared light through the middlezone 41 or the outer zone 42 by use of a wavelength selective film. Or,the objective lens 9 is designed to allow the amount of such infraredlight to be dispersed to a plurality of orders of diffracted light.

The objective lens 9 converges the red light 14 transmitting through theinner zone 40 and the middle zone 41 through a transparent substratehaving a thickness of about 0.6 mm of the optical disc 27 such as a DVDor the like. The objective lens 9 is designed to give red lighttransmitting through the outer zone 42 an aberration exceeding 70 mλms,which is the Marechal Criterion, or not to transmit red light throughthe outer zone 42 by use of a wavelength selective film. Or, theobjective lens 9 is designed to allow the amount of such red light to bedispersed to a plurality of orders of diffracted light.

The objective lens 9 converges the blue 2 transmitting through effectivediameter zones thereof, i.e., the three zones 40, 41 and 42 through atransparent substrate having a thickness of about 0.1 mm or less of theoptical disc 26 such as a BD or the like. In other words, the inner zone40 converges the optical beam to all of the CD, DVD and BD, the middlezone 41 converges the optical beam to the DVD and BD, and the outer zone42 converges the optical beam only to the BD.

FIG. 4( a) shows luminous fluxes 43 and 44 of the infrared lighttransmitted through the inner zone 40. For the sake of simplicity, therefraction by the transparent substrate of the optical discs is omitted.

The objective lens 9 is designed to fulfill the following conditions.The infrared light 43, which passes through an inner area of the innerzone 40 including the optical axis, i.e., a central area of the innerzone 40, is converged to focal point F1 with almost no aberration. Bycontrast, the infrared light 44, which passes through an area includedin the inner zone 40 but outer with respect to the central area andcloser to the middle zone 41, i.e., an outer peripheral area of theinner zone 40, is given a high-order spherical aberration so as to beconverged in the vicinity of convergence point F2, which is slightlyaway from the objective lens. It should be noted that an average valueof sums of squares of the aberrations of both the infrared light 43 andthe infrared light 44 passing through the inner zone 40 (RMS value) isset to be equal to or less than 20 mλ, desirably equal to or less than10 mλ, in order not to obstruct reduction of the convergence spot on theoptical disc to the diffraction limit.

The above-provided expression that the infrared light 44, which passesthrough the outer peripheral area of the inner zone 40 is “given aspherical aberration” means, as simply described, that the infraredlight 44 passing the outer peripheral area of the inner zone 40 isconverged at slightly different focal points or a plurality of positionsin accordance with the distance between the point of transmission andthe optical axis.

In general, the objective lens 9 is designed such that the lighttransmitted through the outer peripheral area of the inner zone 40 isalso converged with no spherical aberration. However, one feature ofthis embodiment is to intentionally give a spherical aberration to theouter peripheral area of the inner zone 40.

FIG. 5 through FIG. 7 show waveforms of focusing error signals obtainedwhere the objective lens 9 designed as described above is used.

FIG. 5 shows a waveform of a focusing error signal obtained when a BD isirradiated with blue light from the light source 1. FIG. 6 shows awaveform of a focusing error signal obtained when a DVD is irradiatedwith red light from the light source 12. FIG. 7 shows a waveform of afocusing error signal obtained when a CD is irradiated with infraredlight from the light source 12. In FIG. 5 through FIG. 7, the horizontalaxis represents the defocus amount, i.e., the distance in the opticalaxis direction between the information recording layer and theconvergence spot, and the vertical axis represents the intensity of thefocusing error signal.

Regarding the focusing error signal for the BD in FIG. 5 and thefocusing error signal for the DVD in FIG. 6, the separation between thedefocus amounts at which the dynamic range, i.e., the focusing errorsignal intensity is respectively maximum and minimum is set to about 2μm. Such design can be realized by setting the thickness of the parallelplate 3, the focal distances between the objective lens 9 (25) and thecollimator lens 7, or the like.

While the defocus amount is in the range of about −1 μm to +1 μm, theinclination of the waveform of the focusing error signal for the BD issteep. This means that the sensitivity of the focusing error signal ishigh. Hence, for the BD, a high sensitivity focusing error signal can beobtained to realize high precision focusing control.

Regarding the focusing error signal for the CD in FIG. 7, the separationbetween the defocus amounts at which the dynamic range, i.e., thefocusing error signal intensity is respectively maximum and minimum,namely, a defocus detection range, can be enlarged to about 4 μm. Thisvalue means that the dynamic range of the focusing error signal is wideand is sufficient to start a focus servo loop with certainty. Such awide dynamic range is realized because the objective lens 9 is designedsuch that the infrared light 40 passing through the outer peripheralarea of the inner zone 40 is given a high-order spherical aberration soas to be converged in the vicinity of convergence point F2, which isslightly away from the objective lens 9.

The focusing error signal is detected utilizing that when theconvergence spot on the optical disc is defocused, the position of theluminous flux incident on the light detector 10 (the distance of theluminous flux from the optical axis) changes. In the example shown inFIG. 4( a), the luminous flux is shifted in the optical axis directionby adjusting the manner of giving the spherical aberration. Namely, apart of the luminous flux is given a spherical aberration, whereasanother part of the luminous flux is given a high-order sphericalaberration so as to be converged in the vicinity of the differentconvergence point F2. The focusing error signal obtained from theseparts of the luminous flux is shifted in the defocus direction withrespect to the focusing error signal obtained from the remaining part ofthe luminous flux. Such a shift enlarges the detection range of thefocusing error signal.

Accordingly, with the objective lens 9 according to this embodiment, thefocus servo (control) can be stably started even for a CD causing alarge face movement.

The objective lens 9 may be designed such that the focal distance of theobjective lens for infrared light is longer than the focal distancethereof for blue light. With this structure also, the effect ofdecreasing the defocus detection sensitivity for infrared light so as toenlarge the defocus detection range therefor can be provided. Ingeneral, “focal distance” is defined as a distance between the focalpoint and the principal point of the objective lens 9. This does notindicate that the distance from the surface of the objective lens 9 (onthe light emission side), or from the light emission point, to theconvergence point differs for the red light and the blue light. Asdescribed above with reference to FIG. 2 and FIG. 4, with the objectivelens 9 according to this embodiment, the position of the lightcollection point (focal point) varies in accordance with the wavelengthor the transmission position of the light. Note that FIG. 2 and FIG. 4show the focal point but not the focal distance. Therefore, it cannot beunderstood from FIG. 2 or FIG. 4 that the focal distance of theobjective lens 9 is varied in accordance with the wavelength.

Embodiment 2

FIG. 8 shows a structure of an optical head device 200 according to thisembodiment. Among the elements of the optical head device 200, elementshaving the same functions as those of the elements included in theoptical head 100 (FIG. 1) according to Embodiment 1 bear the samereference numerals and descriptions thereof will be omitted.

Main differences are that the optical head device 200 includes adetection lens 501, and an objective lens 25 in place of the objectivelens 9.

The detection lens 501 is an anamorphic lens designed so as to havedifferent focal distances along two perpendicular axes in order todetect a focusing error signal by the astigmatism method.

The objective lens 25 is designed as follows. Hereinafter, the objectivelens 25 according to this embodiment will be described in detail.

The objective lens 25 has compatibility for BDs, DVDs and CDs. Specificspecifications of the objective lens are shown in Tables 1 through 4.

TABLE 1 Usable wavelength, material refractive index, separation Opticaldisc BD DVD CD Wavelength [nm] 405 658 785 Focal distance [mm] 2.30 2.372.39 Refractive index of 1.63059 1.60981 1.60585 lens Refractive indexof 1.61735 1.57828 1.57203 protective layer Numerical aperture 0.85 0.60.47 Aperture diaphragm [mm] 3.92 2.77 2.20 diameter Object point [mm] ∞−200 138 distance Working distance [mm] 0.900 0.605 0.303 Protectivelayer [mm] 0.0875 0.6 1.2 thickness

As shown in Table 1, the wavelength of the laser light and the numericalaperture (NA) used for performing recording to, or reproduction from,BDs, DVDs and CDs are different. As shown regarding the object pointdistance, laser light is incident on the objective lens as generallyparallel light for performing recording to, or reproduction from, a BD;whereas prescribed converged or diverged light is incident on theobjective lens for performing recording to, or reproduction from, a DVDor a CD.

TABLE 2 Radius of Face Face No. curvature separation Material 0 ∞ Objectpoint Air distance 1 (−1.563) 2.364199 Lens 2 −8.360 0 Air 3 ∞ WorkingAir distance 4 ∞ Protective Polycarbonate layer thickness 5 ∞(Information — recording face)

In Table 2, each face number represents the following: face number 0represents the light source, face number 1 represents a first face 25 aof the objective lens 25, face number 2 represents a second face 25 b ofthe objective lens, face number 3 represents the reference face of theworking distance, face number 4 represents the surface of the protectivesubstrate of the optical disc, and face number 5 represents theinformation recording layer of the optical disc.

Given as the face separation and the material are the separation, andthe material filling the separation, between the face represented by therespective face number and the face represented by the next face number.Regarding the object point distance, the working distance and theprotective substrate thickness, the BD, DVD and CD each have the objectpoint distance, the working distance and the protective substratethickness shown in Table 1.

The first face 25 a and the second face 25 b of the objective lens 25are respectively a face of the objective lens 25 facing the collimatorlens 7 and a face of the objective lens 25 facing the optical disc 26.

Regarding the aspheric shape of the first face 25 a and the second face25 b of the objective lens 25, the distance (sag) Z from the tangentplane on the optical axis in the optical axis direction is representedby the following expression, where h is the distance in a directionperpendicular to the optical axis, R is the radius of curvature on theparaxial, k is the conic constant, and Ai is the aspheric coefficientwhere i=0th to n'th term.

$\begin{matrix}{Z = {\frac{h^{2}/R}{1 + \sqrt{1 - {\left( {1 + k} \right)\left( {h/R} \right)^{2}}}} + {\sum\limits_{i = 0}^{n}{A_{i}h^{2i}}}}} & \left\lbrack {{Expression}\mspace{14mu} 1} \right\rbrack\end{matrix}$

The first face 25 a of the objective lens 25 is divided into an innerzone and an outer zone which are concentrically arranged around theoptical axis. The inner zone and the outer zone have different asphericshapes. The inner zone has a diameter of 2.78 mm, which is generally thesame as the aperture diaphragm diameter of the DVD shown in Table 1.

Table 3 shows the conic constant k and the aspheric coefficient Ai whichdefine the shape of each of the first face 25 a and the second face 25b. “E” represents the power in the case where the radix is 10 and thenumerical figure right to E is the exponent. For example, “E-02”represents ten to the power of minus two (10⁻²).

TABLE 3 Aspheric coefficient of the objective lens Face of the Firstface objective lens Inner zone Outer zone Second face Diameter  2.780 —— R 1.562919E+00 1.918229E+00 −8.360050E+00  k −9.091400E−01 −4.099720E−01  0.0 A0 0.0 3.615405E−02 0.0 A1 0.0 0.0 0.0 A21.289825E−02 3.843053E−02 1.097133E−01 A3 1.481426E−03 −4.039772E−03 −1.133723E−01  A4 1.866669E−04 −1.228919E−03  9.179145E−02 A5−4.535987E−05  4.576172E−04 −4.240251E−02  A6 −3.596199E−06 −1.357717E−04  2.599427E−03 A7 1.184700E−05 7.266720E−05 7.200437E−03 A8−2.211288E−06  −1.160691E−05  −3.508770E−03  A9 0.0 0.0 6.801656E−04 A100.0 0.0 −4.848957E−05 

The objective lens 25 according to this embodiment further includes adiffraction structure on the first face 25 a.

The differential phase detection function v defining the diffractionstructure is represented by the following expression, where h is thedistance in a direction perpendicular to the optical axis, M is thenumber of diffraction order, and ai is the constant where i=second ton'th term.

$\begin{matrix}{\varphi = {M{\sum\limits_{i = 1}^{n}{a_{i}h^{2i}}}}} & \left\lbrack {{Expression}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Table 4 shows the differential phase detection function which definesthe diffraction structure formed on the first face 25 a of the objectivelens 25. For all of the BD, DVD and CD, the diffraction order numberM=1.

TABLE 4 Phase shift function of the objective lens Face of the Firstface objective lens Inner zone Outer zone Diameter 2.780 — a1 0.000000E+00  0.000000E+00 a2 −2.565179E+01 −1.773514E+01 a3−2.129913E+00 −7.267530E+00 a4 −1.193912E+00  1.292995E+00

In the optical head device 200 shown in FIG. 8, the focal distance ofthe collimator lens 7 is 16.1 mm. Regarding the synthesized focaldistance of the detection lens 501 and the collimator lens 7, the focaldistance of the anterior focal line is 42.4 mm and the focal distance ofthe posterior focal line is 46.0 mm.

FIG. 9 shows a focusing error signal for a CD, in the optical headdevice 200 using the objective lens 25, obtained by calculation. Thedefocus detection range of the CD is about 3.9 μm. As in Embodiment 1,it is understood that a wide dynamic range is obtained. Thus, a focusservo loop can be started with certainty.

FIG. 10 schematically shows an optical head device including a paraxiallens 511 in place of the objective lens 25 shown in FIG. 8. Herein, theterm “paraxial lens” means an “ideal lens having a thickness of zero”which is capable of collecting light with no aberration. Regarding theCD, the paraxial lens 511 is assumed to have the same focal distance tothat of the objective lens 25 in FIG. 8.

FIG. 11 shows a focusing error signal for a CD, in the optical headdevice shown in FIG. 10 where the paraxial lens 511 is used as theobjective lens 25, obtained by calculation. According to FIG. 11, thedefocus detection range of the CD is about 3.3 μm.

As is clear from comparing FIG. 9 and FIG. 11, use of the objective lens25 allows the defocus detection range (so-called S-zone) for the CD tobe increased by about 0.6 μm as compared with the result of calculationconducted using the paraxial lens 511. Namely, the defocus detectionrange for the CD can be increased by using the objective lens 25according to this embodiment in place of a conventional objective lensproving no spherical aberration. Accordingly, the optical head deviceaccording to this embodiment can perform focus servo more stably evenfor a CD causing a large face movement.

Embodiment 3

FIG. 12 shows a structure of an optical information apparatus 167 usingthe optical head device according to Embodiment 1 or 2. In thefollowing, the optical head device according to Embodiment 1 is used asan example.

In FIG. 12, an optical disc 26 (or 27 or 28; as applied hereinafter) isput on a turntable 182 and rotated by a motor 164. An optical headdevice 155 according to Embodiment 1 is fluttered by a driving device151 for the optical head device up to a position of a track of theoptical disc where desired information is existent.

The optical head device 155 transmits a focusing error signal or atracking error signal to an electric circuit 153 in correspondence withthe positional relationship between the optical head device 155 and theoptical disc 26. In response to the signal, the electric circuit 153transmits a signal for fine-moving the objective lens to the opticalhead device 155. Based on this signal, the optical head device 155performs focus servo (control) and tracking control on the optical discand thus can read, write (record) or erase information.

The optical information apparatus according to this embodiment uses theabove-described optical head device, and therefore can stably recordinformation to, or reproduce information from, a plurality of opticaldiscs of different recording densities with one such optical headdevice.

Embodiment 4

FIG. 13 shows a structure of a computer (PC) 300 including the opticalinformation apparatus 167 according to Embodiment 3.

The PC 300 includes a calculation device 364 and the optical informationapparatus 167 according to Embodiment 3. The PC 300 forms a computersystem together with a separate output device 361 and a separate inputdevice 365.

The input device 365 is a keyboard, a mouse, a touch panel or the likefor inputting information. The calculation device 364 includes, forexample, a central processing unit (CPU) for performing calculationsbased on information input from the input device or information read bythe optical information apparatus 167. The output device 361 is acathode ray tube, a liquid crystal display device, a printer or the likefor displaying information, such as a calculation result obtained by thecalculation device 364 or the like.

In the case of a laptop PC, the PC 300, the input device 365 and theoutput device 361 are integrated together.

A computer including the optical information apparatus according toEmbodiment 3 or adopting the above-described recording or reproductionmethod can stably record information to, or reproduce information from,different types of optical discs and so is usable for variousapplications.

Embodiment 5

FIG. 14 shows a structure of an optical disc player 321 including theoptical disc apparatus 167 according to Embodiment 3.

The optical disc player 321 includes the optical information apparatus167 according to Embodiment 3, a liquid crystal monitor 320, and adecoder 366. The decoder 366 is a conversion device or a conversioncircuit for converting an information signal obtained by the opticalinformation apparatus 167 into an image or audio data. The liquidcrystal monitor 320 outputs a post-conversion image.

An optical disc player including the optical information apparatusaccording to Embodiment 3 or adopting the above-described recording orreproduction method can stably record information to, or reproduceinformation from, different types of optical discs and so is usable forvarious applications.

The optical disc player 321, when combined with a positional sensor suchas a GPS or the like and a central processing unit (CPU) and alsogeographical data stored on an optical disc (for example, a CD, DVD orBD), is usable as a car navigation system. The display device 320 is notabsolutely necessary and is not an essential element of the optical discplayer 321.

Embodiment 6

FIG. 15 shows a structure of an optical disc recorder 110 including theoptical information apparatus 167 according to Embodiment 3.

The optical disc recorder 110 includes the optical information apparatus167 according to Embodiment 3, the decoder 366, and an encoder 368. Thedecoder 366 is as described above in Embodiment 5. The encoder 368converts image information into information of a format recordable on anoptical disc (for example, a CD, DVD or BD) by the optical informationapparatus.

It is usually preferable that the decoder 366 is provided so that aninformation signal obtained by the optical information apparatus 167 canbe converted into an image or audio data and output to the output device361, because this enables already recorded information to be reproduced.However, it is not absolutely necessary to provide the decoder 366.

The output device 361 is a cathode ray tube or a liquid crystal displaydevice, but may be a printer. The output device 361 is not absolutelynecessary for the optical disc recorder 110.

Embodiment 7

FIG. 16 shows a structure of a vehicle 300 including the optical discapparatus 167 according to Embodiment 3. The vehicle 300 is a train caror an automobile. In the example of FIG. 16, the vehicle 300 is anautomobile.

The vehicle 300 includes a handle 130, a vehicle body 131 having theoptical information apparatus 167 mounted thereon, a GPS unit 132,wheels 133, a power generation section 134, a fuel storage section 135,and a power source 136.

The power generation section 134 generates power for moving the vehiclebody 131. The power generation section 134 is, for example, an engine.The fuel storage section 135 stores fuel to be provided to the powergeneration section 134.

By mounting the optical disc apparatus 167 on the vehicle body 131, aneffect of allowing a user to stably obtain information from, or recordinformation to, various types of optical discs while staying in thevehicle body 131 can be provided.

By additionally providing a changer 138 or an optical disc accommodationsection 139 in the vehicle body 131, it is made possible to use a largernumber of optical discs easily.

By providing the calculation device 164 for processing informationobtained from an optical disc to generate image or audio information, asemiconductor memory 137 for temporarily storing information, or adisplay device 142, it is made possible to reproduce a moving image suchas a movie or the like from the optical disc.

By providing an amplifier 140 and a speaker 141, it is made possible toreproduce audio data or music from an optical disc. By providing apositional sensor such as a GPS unit 132 or the like, it is madepossible to allow the user to learn the current position or the movingdirection of the vehicle 300 from an image displayed on the displaydevice 142 or audio data output from the speaker 141 together withgeographical information reproduced from an optical disc. By providing awireless communication section 140, it is made possible to allow theuser to obtain information from outside to be used complimentarily withthe information obtained from the optical disc.

In Embodiments 5 and 6, neither FIG. 14 nor FIG. 15 shows an inputdevice. For example, a keyboard, a touch panel, a mouse, a remotecontrol device or the like may be provided. In Embodiments 4 through 6,the input device may be separately sold, in which case only an inputterminal may be provided.

INDUSTRIAL APPLICABILITY

An optical head device according to the present invention can performrecording to, or reproduction from, a plurality types of optical discswhich are different in the substrate thickness, usable wavelength,recording density or the like. A compatible optical informationapparatus using such an optical head can handle many standards ofoptical discs including CD, DVD and BD. Accordingly, such an opticalinformation apparatus is applicable to any system for storinginformation on an optical disc, such as a computer, an optical discplayer, an optical disc recorder, a car navigation system, an editingsystem, a data server, an AV component, a vehicle or the like.

1. An optical head device, comprising: a plurality of light sourcesswitchably usable; an objective lens for converging light emitted fromone of the plurality of light sources to an information recording layerof an optical disc; a a light detector for receiving the light reflectedby the information recording layer and outputting an electric signalbased on the amount of the received light; and an optical elementlocated on an optical path on which the light reflected by theinformation recording layer proceeds until being incident on the lightdetector, the optical element giving astigmatism to the lighttransmitting therethrough; wherein: the plurality of light sourcesinclude a first light source for emitting light having a firstwavelength and a second light source for emitting light having a secondwavelength shorter than the first wavelength; a defocus detection rangeof a focusing error signal obtained from an electric signal based on theamount of received light having the first wavelength is wider than adefocus detection range of a focusing error signal obtained from anelectric signal based on the amount of received light having the secondwavelength; the light having the first wavelength reflected by theinformation recording layer and the light having the second wavelengthreflected from the information recording layer are both incident on thelight detector; the light detector performs photoelectric conversion onthe incident light to generate an electric signal for obtaining afocusing error signal; and the light detector receives the lightreflected by the information recording layer and given the astigmatismto generate the focusing error signal by an astigmatism method. 2.(canceled)
 3. (canceled)
 4. The optical head device of claim 1, furthercomprising a rising mirror for turning the light emitted from one of theplurality of light sources in a direction vertical to the optical disc;wherein an optical axis of the light incident on the rising mirror hasan angle of about 45 degrees with respect to a track groove of theoptical disc.
 5. The optical head device of claim 1, wherein the lighthaving the first wavelength emitted from the first light source and thelight having the second wavelength emitted from the second light sourceare both incident on the objective lens.
 6. The optical head device ofclaim 5, wherein: the objective lens includes at least an inner zoneincluding an optical axis and an outer zone surrounding the inner zone,and the inner zone includes a central area including the optical axisand an outer peripheral area outer with respect to the central area; andthe light having the first wavelength passing through the central areais converged to a first position on the information recording layer ofthe optical disc, and the light having the first wavelength passingthrough the outer peripheral area is given a spherical aberration to beconverged to at least one second position, which is different from thefirst position in a direction vertical to the optical disc.
 7. Theoptical head device of claim 6, wherein the light having the firstwavelength passing through the outer peripheral area is converged to aposition farther from the objective lens than the first position by thespherical aberration.
 8. The optical head device of claim 6, wherein:the plurality of light sources further include a third light source foremitting light having a third wavelength shorter than the secondwavelength; wherein the objective lens converges the light, having thefirst wavelength passing through the central area of the inner zone,through a transparent substrate of the first optical disc; converges thelight, having the third wavelength passing through the inner zone andthe outer zone, through a transparent substrate of the third opticaldisc; and converges the light, having the second wavelength passingthrough an effective diameter zone of the objective lens, through atransparent substrate of the second optical disc.
 9. The optical headdevice of claim 6, wherein the inner zone of the objective lens isdesigned such that an average value of sums of squares of theaberrations of the light having the first wavelength passing through theinner zone is equal to or less than 20 mλ.
 10. The optical head deviceof claim 5, wherein a focal distance f1 of the objective lens forconverging the light having the first wavelength is longer than a focaldistance f2 of the objective lens for converging the light having thesecond wavelength.
 11. The optical head device of claim 10, wherein: thelight having the first wavelength reflected by the information recordinglayer and the light having the second wavelength reflected by theinformation recording layer are both incident on the light detector; andthe light detector performs photoelectric conversion on the incidentlight to generate an electric signal for obtaining a focusing errorsignal.
 12. The optical head device of claim 1, further comprising: aparallel plate for reflecting the light having the second wavelengthemitted from the second light source; and an optical element forreflecting the light having the second wavelength reflected by theparallel plate and transmitting the light having the first wavelengthemitted from the first light source.
 13. An objective lens used, in anoptical head device including a plurality of light sources switchablyusable, for converging light emitted from one of the plurality of lightsources to an information recording layer of an optical disc, wherein:the plurality of light sources of the optical head device include afirst light source for emitting light having a first wavelength and asecond light source for emitting light having a second wavelengthshorter than the first wavelength; the objective lens includes at leastan inner zone including an optical axis and an outer zone surroundingthe inner zone; and the light having the first wavelength passingthrough an central area is converged to a first position on theinformation recording layer of the optical disc, and the light havingthe first wavelength passing through an outer peripheral area is given aspherical aberration to be converged to at least one second position,which is different from the first position in a direction vertical tothe optical disc.
 14. The objective lens of claim 13, wherein the lighthaving the first wavelength passing through the outer peripheral area isconverged to a position farther from the objective lens than the firstposition by the spherical aberration.
 15. The objective lens of claim13, wherein in the case where the plurality of light sources of theoptical head device further include a third light source for emittinglight having a third wavelength shorter than the second wavelength, theobjective lens converges the light, having the first wavelength passingthrough the central area of the inner zone, through a transparentsubstrate of the first optical disc; converges the light, having thethird wavelength passing through the inner zone and the outer zone,through a transparent substrate of the third optical disc; and convergesthe light, having the second wavelength passing, through an effectivediameter zone of the objective lens through a transparent substrate ofthe second optical disc.
 16. The objective lens of claim 13, wherein theinner zone is designed such that an average value of sums of squares ofthe aberrations of the light having the first wavelength passing throughthe inner zone is equal to or less than 20 mλ.
 17. The objective lens ofclaim 13, wherein a focal distance f1 for converging the light havingthe first wavelength is longer than a focal distance f2 for convergingthe light having the second wavelength.
 18. An optical informationapparatus, comprising: the optical head device of claim 1; a motor forrotating the optical disc; and a circuit for controlling and driving themotor, an optical lens and the light source based on a signal obtainedfrom the optical head device.
 19. A computer, comprising: the opticalinformation apparatus of claim 18; an input device or an input terminalfor inputting information; a calculation device for performing acalculation based on at least one of information input from the inputdevice and information reproduced by the optical information apparatus;and an output device or an output terminal for displaying or outputtingat least one of information input from the input device, informationreproduced by the optical information apparatus, and a calculationresult obtained by the calculation device.
 20. An optical disc player,comprising: the optical information apparatus of claim 18; and aninformation-to-image decoder for converting an information signalobtained by the optical information apparatus into an image. 21.(canceled)
 22. (canceled)
 23. (canceled)
 24. An optical head device,comprising: a plurality of light sources switchably usable; an objectivelens for converging light emitted from one of the plurality of lightsources to an information recording layer of an optical disc; and alight detector for receiving the light reflected by the informationrecording layer and outputting an electric signal based on the amount ofthe received light; wherein: the plurality of light sources include afirst light source for emitting light having a first wavelength and asecond light source for emitting light having a second wavelengthshorter than the first wavelength; a defocus detection range of afocusing error signal obtained from an electric signal based on theamount of received light having the first wavelength is wider than adefocus detection range of a focusing error signal obtained from anelectric signal based on the amount of received light having the secondwavelength; the light having the first wavelength and the light havingthe second wavelength are both incident on the objective lens; theobjective lens includes at least an inner zone including an optical axisand an outer zone surrounding the inner zone, and the inner zoneincludes a central area including the optical axis and an outerperipheral area outer with respect to the central area; and the lighthaving the first wavelength passing through the central area isconverged to a first position on the information recording layer of theoptical disc, and the light having the first wavelength passing throughthe outer peripheral area is given a spherical aberration to beconverged to at least one second position, which is different from thefirst position in a direction vertical to the optical disc.